1. Field of the Invention
This invention relates to a heating device of the light irradiation type that perform rapid and high-temperature heat treatment by means of light irradiation of an article to be heated.
2. Description of Related Art
Heat treatment of the light irradiation type in the semiconductor production process is performed by a broad range of processes including film formation, diffusion, and annealing. In all of these processes, a semiconductor wafer (simply “wafer” hereafter), which is a plate-shaped workpiece, is heat-treated at a high temperature.
If heat treatment of the light irradiation type is used in this heat treatment, it is possible to rapidly heat and cool the wafer. For example, it is possible to increase the temperature of the wafer to 1000° C. or more in from several seconds to several tens of seconds after irradiation of the wafer begins. If the light irradiation is stopped, it is possible to cool the wafer rapidly.
A heating device of the light irradiation type generally uses multiple incandescent lamps as a heat source. As for the incandescent lamps in which filaments are arranged within light-emitting bulbs made of optically transparent material, the startup of optical output is quick, so the incandescent lamps are suitable for rapid heat-treatment of a workpiece. In the case of halogen lamps, in particular, a cyclical regeneration cycle (halogen cycle) of the halogen and the tungsten filament, which heats and vaporizes that are sealed in the light-emitting bulb, as a result of which there is the advantage of extending the service life without size reduction or blackening of the bulb.
In the event that the material of the wafer is silicon, if an uneven temperature distribution occurs in the wafer when the wafer is heated to 1050° C. or more, a phenomenon called slip, which is a defect of crystal transition, occurs in the wafer, and the wafer being heat treated is deteriorated and becomes an inferior good. Therefore, in the event that the wafer is heat-treated using a heating device of the light irradiation type, it is necessary to heat the wafer, maintain it at a high temperature, and cool it so that the temperature distribution is uniform across the full surface of the wafer. It is the same if the wafer is heated for the purpose of film formation. That is, in order to form a film of uniform thickness across the full surface of the wafer, the wafer must be heated so that the temperature distribution of the wafer is uniform.
It is thought that, in the event that the physical properties are uniform across the full surface of the wafer, the wafer temperatures will be uniform if light illumination is performed so that irradiance is uniform across the full surface of the wafer. In reality, however, even if light irradiation is performed under that sort of irradiation conditions, temperatures are lower at the periphery of the wafer and an uneven temperature distribution occurs on the wafer. In the event that the wafer is heated to 1050° C. or more, as described above, an uneven wafer temperature distribution is created and slip will occur on the wafer.
The reason that temperatures are lower at the periphery of the wafer is that heat is radiated by the wafer periphery area, such as the sides of the wafer and the portions near the sides of the wafer. Consequently, it is necessary to compensate for the temperature reduction caused by heat radiation from the wafer periphery area in order to make the temperature distribution uniform across the full surface of the wafer. For example, light irradiation can be performed so that irradiance on the surface of the wafer periphery area is greater than the irradiance on the central surface of the wafer.
On the other hand, one method proposed to prevent lower temperatures at the periphery of the wafer is the method of surrounding the periphery of the wafer with an auxiliary piece having the same thermal capacity as the wafer. Such auxiliary piece is generally called a guard ring.
In the event that the thermal capacity of the guard ring placed around the periphery of the wafer is the same as the thermal capacity of the wafer, the wafer and guard ring can be regarded as unified virtual plate-shaped body. In that event, the peripheral area of the wafer will not be the peripheral area of the virtual plate-shaped body, and so heat radiation from the peripheral area of the wafer will not occur. For that reason, the temperatures of the peripheral area of the wafer will not be lowered. In other words, by using a guard ring as described above, it is possible to compensate for heat radiation from the peripheral area of the wafer, and to make the temperatures of the wafer uniform.
Now, because the guard ring is set to enclose the periphery of the wafer, the guard ring often is given the additional function of holding the edge of the wafer and so is used as a wafer holder.
In reality, however, it is difficult to manufacture a guard ring so that it can be regarded as a single unit with the wafer (that is, so that their thermal capacities are equal). The reasons for that are shown below.
(a) For the guard ring and the wafer to have the same thermal capacity, the material of the guard ring should be the same as the material of the wafer. For example, if the wafer is a silicon wafer, the material of the guard ring should be silicon (Si). However, when silicon is repeatedly subjected to great temperature changes it deforms and is unable to fulfill the function of a guard ring.
(b) To avoid the problem of deformation, guard rings are often made of silicon carbide (SiC). Silicon carbide has a slightly greater specific heat than silicon, but the difference is not great. Nevertheless, silicon carbide is hard to machine, and because of machining problems (yield) the thickness cannot be less than 1 mm, so it will be thicker than the wafer thickness of 0.7 to 0.8 mm. Because of the different specific heats of silicon and silicon carbide and the different thicknesses of the wafer and the silicon carbide guard ring, the thermal capacity per unit area of the guard ring, when heated to a high temperature, may be as great as 1.5 times that of the wafer.
Accordingly, in order to make the guard ring function to compensate for the temperature drop in the peripheral area of the wafer, it is necessary to cancel out the influence of the difference of thermal capacities between the wafer and the guard ring. Concretely, it is necessary to provide light irradiation so that the irradiance on the guard ring is greater than the irradiance on the wafer.
Because of the situation described above, it is essential that a heating device of the light irradiation type have the function of light irradiation in which the irradiance in the irradiated region can be set at will. By using a heating device of the light irradiation type having such a function, it is possible to provide light irradiation so that the irradiance on the guard ring is greater than the irradiance on the wafer, as described above. A heating device of the light irradiation type having that function is explained below.
In this heating device of the light irradiation type, the heat source comprises multiple incandescent lamps. The multiple lamps are controlled by dividing them into a number of control zones (lamp groups) and setting the distribution of the irradiance in the irradiated region to the prescribed distribution. For example, the heating device of the light irradiation type described in JP-A-S62-20308 (of 1987) has as its light irradiation means, which is the heating means, multiple straight-bulb halogen lamps arrayed in parallel. These halogen lamps are divided up with several in each of a number of lamp groups; each lamp group is a control unit, and the thermal output from each lamp group can be controlled independently. Concretely, the temperature at multiple points on the workpiece is first detected by a radiation thermometer. Then the control units are controlled so that the temperatures on the workpiece will be uniform.
The heating device of the light irradiation type described in JP-A-S63-260127 (of 1988) and corresponding U.S. Pat. No. 4,859,832 has as its light irradiation means, which is the heating means, multiple infrared lamps arrayed concentrically in rings of differing diameters. These lamps are also controlled by dividing them up into a number of control zones (lamp groups).
Concretely, for various workpiece temperature distribution patterns determined in advance, data on how the lighting of the multiple infrared lamps can be controlled to obtain the desired temperature uniformity on the workpiece is collected. Based on that data, the controller of the heating device of the light irradiation type stores a table of infrared lamp lighting control patterns for individual temperature distribution patterns.
Then, during heat processing, two or more points on the workpiece are measured with a radiation thermometer to learn the temperature distribution pattern. The controller searches the stored table for the temperature distribution pattern that is closest to the measured temperature distribution pattern. Based on the infrared lamp lighting control pattern that corresponds to the temperature distribution pattern resulting from the search, the lamp groups are controlled to make the workpiece temperatures uniform.
A concrete explanation of an example of light irradiation of the guard ring at a radiation irradiance greater than that on the wafer, by means of zone control, is given below. Now, for ease of understanding, the emissivity distribution of the surface of the wafer that is the workpiece is taken to be uniform. That is, the temperature of the light-irradiated wafer is considered to be proportional to the irradiance on the wafer surface.
As semiconductor integrated circuits have become finer and more highly integrated, the circuit structures formed on wafers have become finer in the direction of wafer depth as well. That is, thin film structure has advanced as a part of this circuit structure. In the process of high-temperature heat treatment of wafers, on the other hand, there are times when it is necessary for the temperature attained by the wafer to exceed 1000° C. Following ion implantation, for example, a high-temperature heat treatment process of that description is used to activate the impurities that have been driven in.
The speed of heat diffusion of the impurities increases at such high temperatures. For that reason, if the wafer is at a high temperature for a long period, the impurity ions will diffuse in the direction of wafer depth, and it will not be possible to maintain the thin film structure mentioned above.
Therefore, heating equipment that realizes abrupt heating and abrupt cooling processes called spike annealing has been developed in recent years. With spike annealing there is no maintaining at a fixed temperature; once the target temperature (the temperature necessary for activation) is reached, cooling follows immediately. In other words, the period during which the wafer is at a high temperature is as short as possible, and the progress of impurity ion diffusion in the direction of wafer depth is suppressed.
To accomplish this, there is a very strong desire to make the rate of temperature rise in the spike annealing process very rapidly and to bring the temperature of the wafer or other workpiece to the target temperature in a short period.
When the spike annealing process is carried out using the heating device of the light irradiation type described above, the irradiance of irradiated light on the surface of the workpiece is great, and so a rapid rate of temperature rise is realized. Here, in the case of a heating device of the light irradiation type having a light irradiation means that comprises multiple straight-bulb halogen lamps arrayed in parallel, reducing the spacing between the lamps will increase the irradiance of the irradiated light on the surface of the workpiece.
Here, if the workpiece is plate-shaped like a wafer and a prescribed size is set, the prescribed number of lamps will be the number that can be arrayed in parallel facing the workpiece. When the irradiance of irradiated light on the surface of the workpiece in such a situation is inadequate, it will be necessary to supply greater power in the halogen lamps or other incandescent lamps that comprise the light irradiation means, and thus increase the luminous energy of the light emitted from the incandescent lamps.
FIGS. 28(a) & 28(b) are diagrams for explaining the distribution of the irradiance of a workpiece (wafer) 106 and a guard ring 105 when the workpiece 106 and the guard ring 105 are irradiated by a light irradiation means that comprises multiple straight-bulb lamps 101 and a wave-shaped mirror 102.
In FIGS. 28(a) & 28(b), the light irradiation means comprises multiple straight-bulb halogen lamps 101 arrayed in parallel. Above the lamps 101 there is a wave-shaped mirror 102 that is a reflecting mirror. The reflective surface of the wave-shaped mirror 102 has multiple concavities arrayed in parallel.
Each of the straight-bulb halogen lamps 101 is partially enclosed by one of the concavities. The concavities of the wave-shaped mirror 102 are designed so that the light reflected by the concavities will have a degree of directionality. That is, the spread of the light reflected by the concavities of the wave-shaped mirror is less than the spread of the light from the lamps 101 that directly irradiates the workpiece 106.
Now, the wafer or other workpiece 106 is generally accommodated in a treatment chamber. The light radiated by the light irradiation means passes through an optically transparent window part 104 that is located in the treatment chamber, and irradiates to workpiece 106.
FIG. 28(a) shows a case in which the multiple straight-line lamps 101 are divided into two lamp groups (zones): lamp group 101-1 for irradiation of the guard ring and lamp group 101-2 for irradiation of the wafer. Here, the lamp group 101-1 for irradiation of the guard ring is the group located above the guard ring 105. The lamp group 101-2 for irradiation of the wafer, on the other hand, is the lamp group located above the workpiece 106 (also called the “wafer” hereafter).
Here, in order to light-irradiate the guard ring 105 with greater irradiance than the workpiece 106, the individual zones (lamp groups) are controlled so that the energy supplied to each lamp 1 that belongs to the lamp group 101-1 for irradiation of the guard ring is greater than the energy supplied each lamp that belongs to the lamp group 101-2 for irradiation of the wafer. As a result, the intensity of the light emitted from the lamps belonging to the lamp group 101-1 for irradiation of the guard ring is greater than the intensity of the light emitted from the lamps belonging to the lamp group 101-2 for irradiation of the wafer.
As is clear from FIG. 28(a), using the zone control described above it is possible to light illuminate the guard ring 105 with greater irradiance than the workpiece 106.
Nevertheless, the irradiance on a part (the edge) of the workpiece 106 is greater than the irradiance at the center of the workpiece, and distribution of the irradiance uniformity of the workpiece 106 cannot be realized. When the emissivity distribution of the surface of the workpiece is uniform, the temperature distribution of the workpiece 106 will be uneven if the distribution of the irradiance on the surface of the workpiece is uneven.
The reason the distribution of the irradiance of the workpiece 106 is uneven is that the irradiated light from the lamp group 101-1 for irradiation of the guard ring also irradiates the peripheral area of the workpiece. In other words, the light emitted from the lamps 101 is diverging light, and so if the workpiece 106 is not close to the lamps 101, the light from the lamp group 101-1 for irradiation of the guard ring, which is located above the guard ring 105, will irradiate the peripheral area of the wafer as well. The intensity of the light emitted from lamps belonging to the lamp group 101-1 for irradiation of the guard ring is of greater intensity than the light emitted from lamps belonging to the lamp group 101-2 for irradiation of the wafer, and so the irradiance at the edge of the workpiece 106 is markedly greater than the irradiance on the center part of the wafer.
In the event that the wafer 106, which is the workpiece, is heated to a temperature of 1050° C. or greater, the uneven irradiance part indicated by the slanting lines in FIG. 28(a) will grow larger; the unevenness of temperature distribution will be marked and slip will occur in the wafer 106.
The uneven irradiance part of the wafer 106 mentioned above can be reduced by increasing the number of zones. FIG. 28(b) is an example where there are 3 zones (lamp groups). In the example shown here, the lamp group 101-2 for irradiation of the wafer is divided into group I and group II. Group II is located above the region where the uneven irradiance part occurred in FIG. 28(a). Group I, on the other hand, is located above the region of the central part of the wafer. Now, the lamp group 101-1 for irradiation of the guard ring is called “group III” hereafter.
The idea of zone control shown in FIG. 28(b) is as follows. For light irradiation of the guard ring 105 with a greater irradiance than the workpiece 106, the energy supplied to the lamps 101 belonging to group III is set to be greater than the energy supplied to the lamps belonging to group I and group II. Further, the light irradiated from group III also irradiates the peripheral area of the wafer, and so to counteract that portion, the energy supplied to the lamps belonging to group II is set to be less than the energy supplied to the lamps belonging to group I. In other words, the energy supplied to the lamps belonging to the various groups decreases in the order, group III, group I, group II.
As is clear from FIG. 28(b), by means of zone control as described above, it is possible to provide light irradiation of the guard ring 105 with greater irradiance than the workpiece 106. Further, it is possible to control the unevenness of distribution of the irradiance that arises from irradiation of the peripheral area of the wafer by the irradiating light from the lamp group 101-1 for irradiation of the guard ring, which is diverging light. Therefore, the temperature distribution of the wafer has better uniformity.
As stated above, zone control by means of at least three lamp groups is necessary in order to provide light irradiance of the guard ring 105 with greater irradiance than the workpiece 106, and to make the irradiance distribution of the surface of the wafer roughly uniform. In reality, the distribution of the irradiance on the workpiece is influenced by a number of factors, including the distance between the workpiece 106 and the light irradiation means (the multiple straight-bulb lamps 101 and the wave-shaped mirror 102 in FIGS. 28(a) & 28(b)), the shape of the reflecting mirror (the wave-shaped mirror 102 in FIGS. 28(a) & 28(b)), and the amount of overlap of light from the various zones (lamp groups). For that reason, zone control can be more complex, and the number of lamp groups is normally greater than three.
Now, the explanation of zone control shown in FIGS. 28(a) & 28(b) is applied in a single dimension perpendicular to the axial direction of the bulbs of the straight-bulb lamps 101. For example, with regard to the axial direction of the bulbs of the straight-bulb lamps, there is only one lamp present in the irradiation region, and so it is not possible to implement zone control. Therefore, in order to apply zone control to the full surface of the wafer (that is, in two dimensions), it is usual to rotate the wafer during light irradiation.
FIGS. 29(a) & 29(b) are diagrams showing the schematic structure of the light irradiation means of a heating device of the light irradiation type suited to the spike annealing process, and the distribution of the irradiance on the wafer, which is the workpiece, and the guard ring when the wafer and guard ring are irradiated by this light irradiation means.
In FIG. 29(a), the light irradiation means comprises multiple straight-bulb halogen lamps 101 arrayed in parallel. Here, in order to make the spacing between lamps as small as possible to suit the spike annealing process, a flat mirror 112 must be adopted as the reflecting mirror placed above the lamps. That is because it would be difficult to narrow the spacing between lamps if, as in FIG. 28(a) & 28(b), a wave-shaped mirror 102 were used as the reflecting mirror.
FIG. 29(a) is, like FIG. 28(b), an example of providing 3 zones (lamp groups). That is, the zones comprise group I located above the central region of the wafer, which is the workpiece, group II located above the peripheral area of the wafer, and group III, which is the lamp group used to irradiate the guard ring.
Control of the individual zones is performed, as in FIG. 28(b), as described below. In order to do light irradiation with irradiance on the guard ring 105 that exceeds the irradiance on the wafer that is the workpiece 106, the energy supplied to the lamps belonging to group III is set to be greater than the energy supplied to the lamps belonging to group I and group II. Further, the light irradiated from group III also irradiates the peripheral area of the wafer, and so to counteract that portion, the energy supplied to the lamps belonging to group II is set to be less than the energy supplied to the lamps belonging to group I.
In other words, the energy supplied to the lamps belonging to the various groups decreases in the order, group III, group I, group II.
As is clear from FIG. 29(a), unlike the case shown in FIG. 28(b), the distribution of the irradiance on the surface of the workpiece 106 is not uniform. This is thought to be because of the following reasons.
As stated above, it is necessary to narrow the lamp spacing of the lamps 101 that make up the light irradiation means as much as possible in order to suit the spike annealing process. A flat mirror 112 is used as the reflecting mirror for that reason. However, the flat mirror, unlike the wave-shaped mirror, does not have concave surfaces. Therefore, the light reflected by the flat mirror, unlike the light reflected by the wave-shaped mirror, does not have a degree of directionality.
Further, the length of the optical path of light from a lamp 101 that is reflected by the flat mirror 112 and arrives at the workpiece (wafer) 106 is longer than the optical path of the light from a lamp 101 that irradiates the wafer 106 directly, and so the spread on the wafer 106 of reflected light from the flat mirror 112 is greater than the spread of direct light from the lamp 101.
Accordingly, there is an increase in the proportion of light emitted from each zone that irradiates parts of the regions irradiated by light emitted from other zones. In other words, the separation of zones deteriorates.
As described above, in order to provide light irradiation so that the irradiance on the guard ring 105 exceeds the irradiance on the workpiece 106, the energy supplied to lamps 101 belonging to group III is set to be greater than the energy supplied to the lamps 101 belonging to group I and group II. Accordingly, the intensity of light emitted from the lamps 101 belonging to group III is greater than the intensity of light emitted from the lamps 101 belong to group I and group II. The effect of the light irradiated on the peripheral area of the workpiece 106 from the lamps 101 belonging to group III is particularly great.
Consequently, it is thought that even if the zone control shown in FIG. 28(b) is adopted in the example shown in FIG. 29(a), the distribution of the irradiance on the surface of the wafer (workpiece) will be uneven, unlike the case shown in FIG. 28(b). As mentioned above, when the emissivity distribution of the wafer surface is uniform, if the distribution of the irradiance on the wafer surface is uneven the temperature distribution of the wafer will be uneven as well. In the event that the wafer is heated to a temperature of 1050° C. or higher, the unevenness of the temperature distribution will be marked when the unevenness of irradiance is great, and slip will occur in the wafer 106.
In such a case, it is thought that it is possible to improve the distribution of the irradiance on the wafer surface by increasing the number of individually controlled zones. However, the control system that controls the zones becomes more complex to the extent that the number of zones is increased, and in practice this is often unrealistic.
Shortening the distance between the lamps 101 that make up the light irradiation means and the workpiece 106, as shown in FIG. 29(b), can be considered as a way to handle the problem described above.
That is, shortening that distance shortens the optical path of the light from a lamp 101 that reaches the workpiece 106 after reflection by the flat mirror 112, and shortens the optical path of light from a lamp 101 that directly irradiates the workpiece 106. By this means, the light from a lamp 101 that reaches the workpiece 106 after reflection by the flat mirror 112 and the light from a lamp 101 that directly irradiates the workpiece 106 both arrive at the workpiece 106 without spreading too much. As a result, the proportion of light emitted from each zone that irradiates parts of the regions irradiated by light emitted from other zones is reduced, and zone separation improves.
Using the constitution shown in FIG. 29(b), zone separation is improved relative to the constitution shown in FIG. 29(a). That is, it is thought possible to improve the uniformity of the distribution of the irradiance on the surface of the workpiece 106 without increasing the number of zones.
In reality, however, the uniformity of the distribution of the irradiance on the surface of the workpiece 106 is not good, as is clear from FIG. 29(b).
In other words, to the extent that the distance between the lamps 101 that make up the light irradiation means and the wafer that is the workpiece 106 is shortened, the light from a lamp 101 that reaches the workpiece 106 after reflection by the flat mirror 112 and the light from a lamp 101 that directly irradiates the workpiece 106 both arrive at the workpiece 106 without spreading too much, and so it becomes impossible to ignore the effect of the non-light-emitting areas among the lamps 101.
Conventionally, to the extent that the distance between the lamps 101 that make up the light irradiation means and the wafer that was the workpiece 106 was long, the light that arrived from the lamps 101 either directly or indirectly by way of the flat mirror 112 spread on the workpiece, and although the degree of uniformity of the distribution of the irradiance on the workpiece was not good, it averaged out to some extent. Therefore, the effect of the non-light-emitting areas among the lamps was reduced.
Nevertheless, although the constitution shown in FIG. 29(b) improves zone separation, there is almost no averaging of irradiance; a relatively sharp drop in irradiance (this relatively sharp drop in irradiance is called “ripple” hereafter) is created in the region on the wafer 106 corresponding to the non-light-emitting areas among the lamps 101.
Therefore, it is difficult, as shown in FIG. 29(b), to improve the uniformity of the distribution of the irradiance on the surface of the wafer 106 even by shortening the distance between the lamps 101 that constitute the light irradiation means and the wafer 106, which is the workpiece, and exercising zone control.
As a measure to overcome the problems of the arrangement that is shown in FIG. 29(b), it is possible to shorten the distance between the lamps 101 that constitute the light irradiation means and the wafer 106, which is the workpiece, and to use a flat diffusion mirror 122 rather than a flat mirror 112 as the reflecting mirror, as shown in FIG. 30(c). In other words, one can attempt to minimize ripple by assuring zone separation with respect to the light emitted from the lamps 101 that irradiates the wafer 106 directly and using diffused light that reaches the wafer 106 from the lamps 101 after reflection by a diffusion mirror.
However, using the constitution shown in FIG. 30(c), it is possible to reduce ripple to some extent but it is not possible to ignore the effect of uneven irradiance resulting from diffused light. That is, the energy supplied to the lamps 101 belonging to group III is set to be greater than the energy supplied to the lamps belonging to group I and group II.
Accordingly, the intensity of light emitted from the lamps 101 belonging to group III is greater than the intensity of light emitted from the lamps belonging to group I and group II. In particular, the distance between the lamps 101 that constitute the light irradiation means and the wafer 106, which is the workpiece, is short, and so the intensity of the diffused light that reaches the wafer 106 from the lamps 101 after reflection by the diffusion mirror 122 is increased. As shown in FIG. 30(c), this diffused light spreads and irradiates the peripheral area of the wafer. Ultimately, as in the case of the constitution shown in FIG. 29(a), the uniformity of the distribution of the irradiance on the surface of the wafer is not good.